High-port-count interleavers provide network design options

The technique of interleaving wavelengths is an effective way to narrow e×isting 200- or 100-GHz DWDM systems down to 50-, 33-, or 25-GHz channel spacing. To accomplish this goal, silica-on-silicon planar lightwave circuit (PLC) interleavers compete with the traditional cascade of individual 1 × 2 interleaver fiber-based technology. In addition to cost and size reduction, better reliability, and improved manufacturability, PLCs present a scalable platform with improved chromatic dispersion (CD) and polarization-mode dispersion (PMD) crucial for 10- and 40-Gbit/s systems.

In interleavers, the output channel spacing (the N in "1×N") increases as the number of ports increase. Traditional 1×2 interleavers sequentially demultiplex a high-channel-count incoming wavelength stream into two lower- channel-count streams with a double-wide channel spacing (see Fig. 1). Higher port-count ("highport") 1×4 interleavers sequentially demultiplex the incoming channel into four output ports. Thus, the 50/100-GHz 1×2 interleaver splits 50-GHz spaced channels into two sets of 100-GHz spaced channels, the 25/100-GHz 1×4 interleaver into four sets of 100-GHz spaced channels, and the 25/200-GHz into eight sets of 200-GHz spaced channels. Interleavers are bi directional and are also used in the opposite way, as multiplexers.

The highport interleavers achieved by silica-on-silicon PLCs feature wideband transmission with a flat top. In contrast to fiber-based technology in which the insertion loss (IL) is highly dependent on the number of ports, the PLC is not. Thus, the 1×2, 1×4, and 1×8 PLC-based interleavers all result in the same IL. The IL for the flat-top PLC interleaver is typically 4 dB within the 0.5-dB bandwidth. The bandwidth and IL are interdependent and are customized during the mask design phase. The wider the passband, the higher the insertion loss.

For example, a 25/100-GHz 1×4 interleaver with an IL of 4 dB has at least ±6 GHz of bandwidth around the ITU grid at 0.5 dB. A ±7 GHz bandwidth leads to a 4.5-dB IL. Thus, when considering performance, there is a tradeoff between passband and IL. The channel isolation is at least 25 dB and the return loss greater than 50 dB. The ripple of the slope within the specified passband defines the flatness of the filter and is always less than 0.5 dB/nm.

The right technology platform is crucial for limiting the dispersion in 10- and 40-Gbit/s systems. Accurate, high-resolution measurement equipment and a good understanding of the involved physical phenomenon are essential for predicting dispersion performance during manufacturing. Group delay (GD) and differential group delay (DGD) are the main parameters for calculating PMD and CD.

The group delay is the difference in arrival time between wavelengths. The differential group delay is the difference in arrival time between the principal polarization states. The DGD and its root-mean-square value, the PMD, exhibit information on several states of polarization traveling within the device. The GD and its derivative, the CD, show how the phase changes with wavelength. The PMD is due to the birefringence of the waveguide and leads to pulse broadening.

For PLC interleavers, the polarization mode dispersion is lower than 0.5 ps. For PLC interleavers, chromatic dispersion is lower than ±5 ps/nm, three times less than that of fiber-based technology. Thus, compared to the conventional fiber-based technology, the silica-on-silicon PLC is the most suitable technology platform for the design of interleavers for 10-Gbits/s and the upcoming 40-Gbit/s technology.

Highport interleavers can be used to upgrade existing DWDM systems. Many system manufacturers propose upgrading existing 100-GHz systems to new 25-GHz systems, but to do this, a 25/100-GHz interleaver is required to split 160 channels of 100-GHz spacing into four sets of 40 channels with 25-GHz spacing. Conventional fiber-based interleavers require the cascade of three discrete components: one 25/50-GHz 1 × 2 interleaver with two 50/100-GHz 1 × 2 interleavers (see Fig. 2). The PLC-based 1×4 interleaver limits the number of connections and avoids central frequency misalignments. Thus, the use of a single interleaver instead of three drastically reduces the cost and the footprint.

For example, a 1×4 PLC interleaver is four times smaller than a fiber-based package with better optical performance (see table). The advantages of the PLC-based highport interleaver become even greater for 8-port applications, such as the 25/200-GHz interleaver. This single-device PLC-based interleaver replaces the cascade of seven discrete fiber-based components-one 25/50-GHz, two 50/100-GHz, and four 100/200-GHz interleavers. Moreover, a silica-on-silicon PLC platform allows the integration of multiplexer/demultiplexer and variable optical attenuator (VOA) arrays. For example, a compact device that integrates one 25/100-GHz 1×4 interleaver with four 40-channel 100-GHz arrayed waveguide (AWG) multiplexers on the same chip is available for 160-channel OC-192 DWDM systems.

The 4×4 PLC interleaver is also used to upgrade reconfigurable optical add/drop modules (ROADMs). The incoming light is first separated into the S-, C-, or L-bands and then into 400-GHz-spaced bands using successively two stages of coarse WDM demultiplexers. The dropping or adding of these bands is accomplished with switches. The proposed upgrade includes an interleaver that demultiplexes each 400-GHz band into four 100-GHz output ports (see Fig. 4). A unique interleaver is required to achieve this functionality. Thus, the use of N × N interleavers reduces by a factor of four the number of variable optical attenuators and switches required. Furthermore, N×N cyclic interleavers with a loop-back configuration with switches are used as low-cost ROADMs.¹

A loop-back configuration including delay lines is useful in optical code-division multiplexing (OCDM). Delay lines, as their name implies, temporally separate wavelengths. The OCDM is attractive because it can offer routing operations in a single channel without optical switches.

Finally, because of low dispersion and good isolation properties, PLC-based highport interleavers already used in 10-Gbit/s systems can enable the deployment of 40-Gbit/s optical networks. They are easily implemented and are used in complex signal processing functionalities with promising performance. Future DWDM systems will use highport interleavers in combination with multiplexers/demultiplexers, VOA arrays, amplifiers, and switches because the PLC platform has already proven its high integration capabilities.

Jérome Prieur is vice president sales and business development at OpsiTech US, 159 Summit Ave., 2F, Summit, NJ 07901. Gregory Pandraud is design senior manager and Mikael Colin is design engineer at OpsiTech S.A., 15 rue des Martyrs, 38000 Grenoble, France. Borja Vidal is an engineer at the Universidad Politécnica de Valencia, 46022 Valencia, Spain. Jérome Prieur can be reached at [email protected].